INFLUENCE OF WATER RELATIONS ON THE LIMITED EXPANSION OF PINUS-MONOPHYLLA INTO ADJACENT CERCOCARPUS-LEDIFOLIUS COMMUNITIES IN THE CENTRAL GREAT-BASIN

We investigated the influence of plant water relations on the limited expansion and dominance of Pinus monophylla (an arid land coniferous t ree) into adjacent Cercocarpus ledifolius (an evergreen nonconiferous shrub or small tree) communities. Water relations and growth of both s pecies were monitored during 2 growing seasons on a P. monophylla domi nated, alluvial fan (2070 m) and a C. ledifolius dominated, exposed ri dge (2745 m). In 1990, the influence of supplemental water at the lowe r site was investigated. Carbon isotope composition was determined fro m material collected in August 1990. Predawn and midday xylem pressure potentials and transpiration rates suggested that P. monophylla was m ore water-use efficient than C. ledifolius. Cercocarpus ledifolius had higher transpiration rates throughout the growing season and reached lower xylem pressure potentials late in the season. This suggests C. l edifolius is better able to take advantage of available water for rech arging plant moisture and rapid water use. Supplemental water s only e ffect was a significant increase in C. ledifolius transpiration. Carbo n isotope ratios (partial derivative C-13) were more negative for C. l edifolius than P. manophylla and with increasing soil water availabili ty. Lower rates of growth and water-use patterns by P. monophylla are not an advantage in the C. ledifolius zone and contribute to its limit ed expansion into this zone.